As the data areal density in hard disk drive (HDD) increases, critical dimensions in the write heads are required to be made in smaller sizes in order to be able to write small media bits. However, as the main pole dimensions (track width and down-track thickness) shrink, its writability degrades. To improve writability, new technology is being developed that assists writing by either increasing the effective write field generated by the heads or by heating up the media to lower the coercivity in the media near the area when the transition is written. Two approaches currently being investigated are heat assisted magnetic recording (HAMR), and microwave assisted magnetic recording (MAMR), that is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008). Although MAMR has been in development for a number of years, it is not shown enough promise to be introduced into any products yet. In particular, a difficult challenge is to find a spin torque oscillator (STO) film that is thin enough to fit into the small write gap required for state of the art products while providing a high magnetic moment in the oscillation layer to generate a sufficient radio-frequency (RF) field for the microwave assist effect.
Spin transfer (spin torque oscillator or STO) devices are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-non-magnetic spacer-ferromagnetic multilayers. When a spin-polarized current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the magnetic moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high.
In a PMR writer, the main pole generates a large local magnetic field to change the magnetization direction of the medium in proximity to the writer. By switching the direction of the field using a switching current that drives the writer, one can write a plurality of media bits on a magnetic recording medium. A new “assist” technology is needed to controllably boost or reduce the MP field on the magnetic medium, and that does not rely on generating a RF field or providing a heating mechanism since MAMR and HAMR have not reached a point of maturity that enables large-scale manufacturing and incorporation into actual devices.